Christopher K.Y. Leung

Experimental determination of the tension-softening relations for cementitious composites

Abstract A novel experimental technique based on the J-integral is employed to experimentally determine the tension-softening (σ-δ) relations in cementitious composites. The σ-δ relation provides information on fracture resistance and could be used for numerical simulations of crack formation and propagation in structures constructed from materials which exhibit “tension-softening” behavior. With this method, no complicated modifications to testing machines are required. In the experiment, two pre-cracked specimens with slightly different notch lengths are used. The corresponding values of load, load point displacement and crack tip separation are simultaneously recorded. From this experimental data, the J-integral as a function of crack tip separation as well as the tension-softening curve can be deduced. These curves also provide a measure of the critical energy release rate. The experimental technique has been applied to mortar. It is suggested that the relatively simple technique can provide reliable material parameters for characterization of fracture resistance in plain and fiber reinforced cementitious composites independent of specimen geometry, size and loading configurations.

Effect of fiber inclination on crack bridging stress in brittle fiber reinforced brittle matrix composites

Abstract The mechanical behavior of brittle matrix composites is strongly affected by the bridging of cracks by fibers. In random fiber composites, fibers can lie at an angle to the crack plane. Under such conditions, the bridging stress for a certain crack opening is governed by various micromechanisms including fiber debonding, fiber bending and rupture as well as matrix spalling. While fiber debonding has been widely investigated, the coupled fiber bending/matrix spalling mechanism has received little attention. In this paper, the fiber bending/matrix spalling mechanism is analyzed by treating the fiber as a beam bent on an elastic foundation with variable stiffness and the possibility of spalling. The foundation stiffness and spalling criterion are derived from a finite element analysis. The bridging stress due to bending alone as well as the total bridging stress are then obtained for the case with brittle fibers. Through this analysis, the effect of various microstructural parameters (such as fiber and matrix moduli, matrix spalling strain and fiber/matrix interfacial friction) on the behavior of random fiber composites can be studied. Prediction of maximum bridging stress for inclined fibers based on the present model is shown to be in good agreement with experimental results.

Investigation of the strain transfer for surface-attached optical fiber strain sensors

The relation between the component strain and the strain on a surface-attached optical fiber is governed by the effectiveness of shear transfer through the adhesive and the polymeric coating(s) on the optical fiber. A classical shear lag model can predict the strain transfer through a soft layer well. However, experiments showed that the results are unsatisfactory for bare fiber with stiff adhesive case. A 3D-FEM is established to model the strain transfer of a surface-mounted strain sensor and it is verified by experiments. Then, it is used to investigate the influence of four geometric parameters of the adhesive: (1) side width, (2) top thickness, (3) bond length, (4) bottom thickness, on the strain transfer. By sensitivity analysis, it is revealed that the bond length and the bottom thickness are dominant factors. Based on finite element results, the parameter of the analytical model is modified to suit stiff layer cases. Important considerations for practical installation of surface-attached optical interferometric and FBG strain sensors will be discussed.

New strength-based model for the debonding of discontinuous fibres in an elastic matrix

The mechanical properties of fibre composites are strongly influenced by the debonding of fibres. When an embedded fibre is loaded from one end, debonding can occur at both the loaded end and the embedded end. Existing theories neglect the possibility of debonding from the embedded end and are thus limited in applications to cases with low fibre volume fraction, low fibre modulus or short fibre length. A new two-way debonding theory for fibre debonding, which can extend the validity of one-way debonding theory to all general cases, is therefore proposed. Based on the new theory, different scenarios of debonding are identified. Comparison is made between results predicted by the new two-way theory and a one-way debonding theory. The relative merits and dismerits of strength-based debonding theories compared to fracture-based debonding theories are briefly discussed.

Fiber optic sensors in concrete : the future?

Fiber optic sensing systems have been successfully developed for many engineering applications. The objective of this review paper is to assess the potential of fiber optic sensors for the monitoring of concrete structures. In this paper, some current applications of fiber optic sensors in concrete structures are first reviewed to demonstrate their applicability in conventional monitoring applications. The advantages of optical sensors over electrical gauges and transducers are also discussed. Then, we will focus on two novel fiber optic sensors for the monitoring of cracking and delamination in concrete structures without requiring a priori knowledge of the damage locations. Through these two examples, it is demonstrated that the proper design of optic sensors can provide useful sensing capabilities unprecedented with conventional sensors. With further research focusing on the development of novel sensing concepts and the reduction of sensor cost, the widespread use of optical sensors in concrete structures can become a reality in the future.

A novel distributed optical crack sensor for concrete structures

Abstract The damage condition of a concrete structure can be assessed through the monitoring of cracks. The objective of the present investigation is to develop a novel optical fiber sensor for the detection of cracks and the subsequent monitoring of their openings. With this sensing concept, (i) no a-priori knowledge of crack location is required, and (ii) a small number of fibers can be employed to detect and monitor a large number of cracks. This paper presents the sensing concept, preliminary experimental results, and a theoretical model for the prediction of optical power loss. The findings of this investigation demonstrate the potential of the novel sensing technique for practical applications.

Microwave curing of Portland cement concrete: experimental results and feasibility for practical applications

Abstract The acceleration of strength development in concrete is beneficial to many operations in construction, such as precasting and pavement repair. In this paper, the effects of microwave on the early (at 4.5 h) and later age (7 day) strength of both mortar and concrete specimens are reported. With microwave curing, type III Portland cement concrete can develop early strength and later age strength that compare very favourably with commercially available rapid hardening concrete as well as concrete containing accelerating admixtures. With the experimental results demonstrating the potential of rapid curing with microwave energy, important issues relating to the development of the microwave curing technique for practical construction applications are discussed.

Effect of Concrete Composition on FRP/Concrete Bond Capacity

External bonding of fiber-reinforced plastics (FRP) to concrete members has been established as an efficient and effective method for structural strengthening and retrofitting. Direct shear test is often employed to study the crack-induced debonding failure in reinforced concrete members flexurally strengthened with FRP composites. In many existing models, the bond capacity (which defines ultimate load capacity of the specimen in the direct shear test) is considered to be strongly dependent on the compressive or tensile strength of the concrete. However, since debonding behavior is affected by interfacial friction due to aggregate interlocking within the debonded zone, the concrete composition should also play an important role in determining the bond capacity. In this study, the direct shear test is performed with 10 different compositions of concrete. The test results indicate that the bond capacity has little correlation with either the concrete compressive or splitting tensile strength. On the other h...

Very high early strength of microwave cured concrete

Abstract A high rate of strength development in concrete can benefit a number of important operations in the construction industry, including concrete precasting and pavement repair. In this study, the acceleration of concrete curing with microwave energy is investigated. Type III Portland cement is employed and both mortar and concrete specimens are made and tested. By applying the microwave energy for only 45 minutes, the compressive strength of concrete at an early age of 4.5 hours can reach 19.2 MPa (2,785 psi) for w c = 0.55 and 27.2 MPa (3,947 psi) for w c = 0.40 . For the 0.55 w/c composition, the 7-day compressive strength reaches 37.4 MPa (5,417 psi), which is higher than that of conventionally cured specimens. Such performance compares favorably with commercially available rapid hardening concrete as well as concrete containing accelerators, showing microwave curing to be a promising technique for practical applications.

Applications of a two-way debonding theory to short fibre composites

Abstract Traditional fibre debonding theories which consider debonding only from the loaded end of the fibre are only applicable to composites with low fibre volume fraction, low fibre/matrix moduli ratio and high interfacial strength/interfacial friction ratio. A two-way debonding theory, which is applicable to all general cases, has recently been developed. In this paper, major findings from the new two-way theory are first summarized. The new theory is compared with traditional theories with respect to the prediction of composite properties. For the fibre pull-out test specimen, where fibre volume fraction is very low, a new method of deriving interfacial bond properties based on one-way debonding theory is presented. For practical composite systems, the significance of employing the new two-way debonding theory is discussed.